1. Field of the Invention
The present invention relates to a supported catalyst and to a fuel cell.
2. Description of the Related Art
Polymer electrolyte membrane fuel cells permit miniaturization and weight reduction of the fuel cell, compared with the other fuel cells and, thus, vigorous effort is being made in an attempt to develop polymer type fuel cells as power sources for use in spaceships. In recent years, extensive research has been made on polymer electrolyte membrane fuel cells as power sources for use in vehicles and mobile equipment.
The polymer electrolyte membrane fuel cell comprises a membrane electrode assembly (MEA) that is used as an electromotive section. The MEA is of a laminate structure comprising an anode diffusion layer (a so-called “current collector”), an anode catalyst layer (a so-called “fuel electrode”), a proton conductive membrane, a cathode catalyst layer (a so-called “oxidizing electrode”) and a cathode diffusion layer (a so-called “current collector”), which are laid one on another in the order mentioned. The catalyst layers contain catalytically active substance, conductive substance, and proton conductive substance, and have fine pores. For a supported catalyst comprising a conductive substance used as a carrier, the catalyst layers contain supported catalyst and proton conductive substance and have fine pores.
Mixed fuel containing organic fuel, such as methanol, and water is supplied to the anode catalyst layer. On the other hand, air (oxygen) is supplied to the cathode catalyst layer. If the mixed fuel and air are simultaneously supplied to the anode catalyst layer and the cathode catalyst layer, respectively, a catalytic reaction takes place on the surface of the catalyst contained in the anode catalyst layer and cathode catalyst layer. The catalytic reaction generates protons in the fuel electrode. The protons migrate into the proton conductive membrane, while electrons migrate into the anode diffusion layer. In the oxidizing electrode, the electrons supplied from the cathode diffusion layer, the protons supplied from the proton conductive membrane, and oxygen react. As a result, electric current flows between the pair of the current collectors. However, the output performance of the fuel cell is low, especially at temperatures lower than 100° C. This inhibits the wide use of the fuel cell. The low output performance of the fuel cell is attributed mainly to the low activity of the catalyst material. This is why an intensive study is now made on catalyst materials for use in the fuel cell.
To improve the catalytic activity of the catalyst material, the alloying with other transition metals and the change of carrier are proposed. Most alloys containing a transition metal have low electrochemical stability and are thus difficult for practical use. On the other hand, oxides or composite oxides have good durability. Oxides are therefore used as carrier materials to support the catalyst material or used as catalyst promoter of the catalyst material. In the latter case, the oxides and the catalysts are both supported on the carrier material such as conductive carbon materials.
For example, JP-A No. 2004-95263 (KOKAI) teaches that a mixture of metal oxide powders and carbon powders that support Pt, or carbon powders that support metal oxide fine particles and Pt fine particles is used as the catalysts of the cathode in which an ozone-containing gas is used as an oxidizing agent.
JP-A No. 9-167620 (KOKAI) aims to provide fuel cell electrode catalysts that suppresse the elution and sintering of a catalytically active metal at a high-temperature cathode atmosphere in which a phosphoric acid electrolyte and oxygen acting as an oxidizing agent are present together. To obtain both high catalytic activity and good durability, metal catalyst particles supported by carbon powders are covered with oxide or hydroxide, either containing Si and at least one element selected from the group consisting of Nb, Ni, Sn, Ta, Ti, and Zr.
The supported catalyst using carbon powders as the carrier material is certainly excellent in conductivity but is low in the catalytic activity. As a result, the fuel cell using the electrode catalyst disclosed in JP-A Nos. 2004-95263 (KOKAI) and 9-167620 (KOKAI) cannot exhibit excellent output performance.
JP-A No. 2004-73991 (KOKAI) aims to convert effectively hydrocarbon compounds into a mixed gas consisting of carbon monoxide and hydrogen, by employing steam reforming reaction using a catalyst supporting a small amount of a cheap metal. The catalyst comprises solid super acid carrier in which a sulfate group or tungsten oxide is supported by zirconium oxide or zirconium hydroxide and at least one metal selected from the Group VIII or Group IB elements of the Periodic Table, which is supported by the solid super acid carrier. Note that the sulfate group or the tungsten oxide is interposed between the oxide or hydroxide of zirconium and the metal. Thus, no interfaces are shared by the oxide or hydroxide of zirconium, the sulfate group or the tungsten oxide, and the metal.
JP-A No. 2003-80077 (KOKAI) discloses a technology of improving the stability of catalyst for removing air pollutants from the exhaust gas of vehicles. This publication teaches that the catalyst comprises base particles formed of single material selected from the group consisting of oxides of Ce, Zr, Al, Ti, Si, Mg, W, Sr and derivatives thereof or a solid solution formed of at least two materials selected from the group consisting of oxides of Ce, Zr, Al, Ti, Si, Mg, W, Sr and derivatives thereof. The publication further teaches that metal particles and a sintering preventing agent are supported by the base particle noted above. The sintering preventing agent is formed of a metal having a melting point not lower than 1,500° C. or an oxide of the metal. The sintering preventing agent prevents the metal particles from being sintered at a high temperature in the vicinity of about 1,000° C., suppressing the decrease in the specific surface area of the metal particles, and thus maintaining high reactivity. Hence, the sintering preventing agent is formed of a single material selected from the group consisting of oxides of Al, Mg, Ca, Ce, Sr, Zn, W and Mo and derivatives thereof, or a solid solution of at least two materials selected from the group consisting of oxides of Al, Mg, Ca, Ce, Sr, Zn, W and Mo and derivatives thereof. Note that the sintering preventing agent formed of the particular materials exhibits a melting point not lower than 1,500° C.
However, using any supported catalyst disclosed in JP-A Nos. 2004-73991 (KOKAI) and 2003-80077 (KOKAI) as the fuel electrode catalyst, both high catalytic activity and high conductivity cannot be attained. Consequently, excellent output performance cannot be achieved. No catalyst materials exhibiting a sufficiently high catalytic activity have not yet been found. This is why Pt—Ru or Pt are mainly used as anode catalyst material and cathode catalyst material. To improve further the output performance of the fuel cell, it is important to develop electrode catalysts having high catalytic activity, high conductivity and high stability.